Process for producing a multilayer high analysis, granular, non-hygroscopic phosphate fertilizer and the product thereof



Apnl 13, 1965 P. D. v. MANNING ETAL 3,178,273

PROCESS FOR PRODUCING A MULTILAYER HIGH ANALYSIS, GRANULAR,NON-HYGROSCOPIC PHOSPHATE FERTILIZER AND THE PRODUCT THEREOF OriginalFiled May 27, 1955 2 Sheets-Sheet 1 .zmyzzs war FZZIW/HMUJV 4 J// Fzkt 1A nl 13, 1965 P. o. v. MANNING ETAL PROCESS FOR PRODUCING A MULTILAYERHIGH ANALYSIS, GRANULAR, NON-HYGROSCOPIC PHOSPHATE FERTILIZER AND THEPRODUCT THEREOF 2 Sheets-Sheet 2 Original Filed May 27, 1955 cqcay 0 w 0w w m. m u n. m

[1511mm mas M0: mania/m4 flaw/w Was/{r mm W 10940 PPM/P 1v Maamm ma/vFig! Z United States Patent PROCESS FOR ?RDUC1N G A IVIULTIIAYER HIGHANALYSIS, GRANULAR, NGN-HYGROSCQPIC PHGSPHATE FERTEJZER THE PRODUQTTI-EREOF Paul D. V. Manning, Pasadena, (Ialiil, and Ira M. Le

Baron, Evanston, Ill., assignors to International Minoral 8; ChemicalCorporation, a corporation of New York Original application May 27,1955, Ser. No. 511,624, now Patent No. 2,976,119, dated Mar. 21, 1961.Divided and this application Aug. 20, 1959, Ser. No. 835,081

(Zlainis. (Cl. 71-34) This application is a division of Manning and LeBaron application Serial No. 511,624, now Patent No. 2,976,119, filedMay 27, 1955, which is a continuation-inpart of Manning and Le Baronapplication Serial No. 151,728, now abandoned, the disclosure of whichis incorporated herein by reference. Related inventions are disclosed inLe Baron applications Serial Nos. 186,850 and 311,950, now Patent Nos.2,709,649, May 31, 1955, and 2,761,775, Sept. 4, 1956, respectively.

This invention relates generally to the production of various phosphaticmaterials from phosphate rock. More particularly, the invention relatesto the production from phosphate rock of feed grade dicalcium phosphateand a high analysis phosphate fertilizer product.

There has developed in recent years a substantial market for dicalciumphosphate of a grade and quality useful as an animal feed supplement. Insuch feed grade dicalcium phosphate, the weight ratio of phosphoruscalculated as elemental phosphorus to fluorine calculated as elementalfluorine must be not less than about 100, and preferably not less thanabout 200. Inasmuch as phosphate rock contains in the natural statesignificant amounts of fluorine, the production of feed grade dicalciumphosphate from phosphate rock has presented a particularly difiicultproblem. This invention, accordingly, embraces a process for themanufacture of feed grade dicalcium phosphate from phosphate rock, whichprocess is particularly advantageous in that it may be integrated on acommercial scale with the production of a high analysis phosphatefertilizer.

Since a substantial portion of the cost of fertilizer must defraytransportation expenses, it is frequently economically unsound for thefarmer to purchase low analysis fertilizers. Fertilizer manufacturers,therefor, generally concurrently with the development of the feed gradedicalcium market have been required to produce increasingly largepercentages of high analysis phosphate fertilizer products. Theconventional high analysis phosphate fertilizer product of the prior artis triple superphosphate in which substantially the entire phosphoruspentoxide content is water-soluble monocalcium phosphate. The additionof such material to the soil at the time of planting gives rise to thecondition wherein the young plants have available, for immediate use,large quantities of phosphorus pentoxide, while the same plants at theirlater stages of development have available less phosphorus pentoxidebecause of the leaching of such values from the soil by rain andirrigation waters. Studies by Nelson et al., Soil Science SocietyProceedings, vol 12, pages 1l3118 (1948), using superphosphatecontaining radioactive phosphorus show that in the case of manyfertilized crops, particularly corn, the phosphate is required inlargest degree by the plant as it approaches maturity, and to a lesserdegree while the plant is in the seedling stage. These studies have alsoshown that plants approaching maturity have been forced to piratephosphorus values from the permanent soil structure because of theabsence of phosphorus derived from the 3,178,278 Patented Apr. 13, 1965applied fertilizer occasioned, at least in part, by previous leaching ofphosphorus values from the applied fertilizer while these same plantswere undergoing their initial growth periods. Thus, the application ofwatersoluble phosphate values to the soil at the time of plantingresults in large quantities of phophorus being presout at a time whenthese large quantities are not needed, and fewer of such values from thefertilizer being present at a subsequent date when large amounts areneeded. This invention accordingly is in part concerned with theproduction from phosphate rock of novel, high analysis phosphatefertilizers which contain a substantial portion of availablewater-insoluble phosphate values and which, therefore, are effective tosupply fertilizing values to plants throughout the normal growingseason. The high analysis phosphate fertilizer products which theinvention embraces can be ammoniated or otherwise modified to provide amore complete high analysis fertilizer.

It is accordingly a primary object of the invention to provide a methodfor the production from phosphate rock of feed grade dicalciumphosphate.

It is more particularly an object of the invention to provide a processfor the calcium defluorination of aqueous solutions offluorine-containing phosphatic materials pursuant to which the extent ofloss of phosphate values to the defiuorination procedure is minimized.

It is an additional object of the invention to provide a method forcontrolling and minimizing a percentage of the phosphate values presentin the phosphate rock treated which are converted into phosphaticfertilizer and to thereby insure a maximum yield of the more valuabledicalcium phosphate.

It is an additional primary object of the invention to provide a methodfor the production from phosphate rock of a high analysis phosphatefertilizer.

It is a further object of the invention to provide a high analysisphosphate fertilizer product effective to provide phosphate values toplants throughout a normal annual growing season.

It is a specific object of the invention to provide a method for theproduction of a novel, multilayered, pelletized phosphate fertilizerproduct.

It is a more specific object of the invention to provide an integratedcommercial process for the concurrent production of a novel highanalysis phosphate fertilizer and of feed grade dicalcium phosphate.

The invention generally finds utility in conjunction with processeswhich entail acidulation of phosphate rock, extraction of thesolubilized phosphate values of the aciclulated rock with an aqueousmedium, and processing the phosphate rich extract so obtained to producea high analysis fertilizer or a feed grade dicalcium phosphate, or both.A representative process of the general type is described in copendingLe Baron application Serial No. 312,519, new Patent S.N. 2,722,472,entitled Process for Producing Phosphate Materials, the disclosure ofwhich is incorporated herein by reference.

In such a process, sulfuric acid is a major item of expense and issubstantially entirely converted to calcium sulfate, a worthlessbyproduct which is discarded. The invention accordingly envisions in oneembodiment the use of an amount of sulfuric acid substantially less thanthat required to convert entirely to phosphoric acid the phosphatevalues of the processed phosphate rock.

Theoretically, the phosphate values of phosphate rock can bewater-solubilized by reaction of the rock with an amount of sulfuricacid requisite only to convert such phosphate values to monocalciumphosphate. The production of ordinary superphosphate entails such aprocedure in which the sulfuric acid may be employed in some cases in anamount up to about 10% in excess of that required to convert tomonocalcium phosphate the phosphate values of the rock. Unfortunately,ordinary superphosphate manufacturing processes yield a product in whichthe calcium sulfate is present in a form exceedingly difiicult toseparate from an aqueous sluriy of such product, such as that formed inan attempt to extract the mo-nocalcium phosphate values thereof withwater.

The present invention accordingly contemplates, in the embodiment hereunder consideration, acidulating of particulate phosphate rock withsulfuric acid in an amount requisite to convert the phosphate values ofthe rock primarily to monocalcium phosphate, agitating or slurryi-ng theacidulated rock with an aqueous medium, and filtering of the slurry soproduced, the entire procedure being carried out under conditions and ina manner effective to produce calcium sulfate and other water-insolublematerials in a form more easily and readily separated from the liquidphase of the acidulated rock slurry.

Pursuant to this facet of the invention, particulate phosphate rock of aparticle size such that at least about 50% by weight, preferably about50% to about 85% by weight thereof, will pass through a 200 mesh screenis reacted with 60% to 70%, preferably 65% to 70%, aqueous sulfuric acidin an amount equal to from about 112% to about 117% of that required toform mono calcium phosphate from the phosphatic materials contained inthe rock and to react with the impurities present therein. Theacidulated rock is then slurried or otherwise contacted, either beforeor after curing, with an aqueous medium to extract the monocalciumphosphate and other water-soluble phosphate values, and the slurry isprocessed to separate undissolved solids, primarily calcium sulfate, inconventional manner. It has been found that by following theseconditions, there is obtained a readily filterable slurry from which thecalcium sulfate and other acid-insoluble and water-insoluble impuritiesmay be readily and expeditiously removed.

Furthermore, the acidulated rock mix is produced in a form and conditionwhich can be readily processed and transported on a commercial basiswhich is not the case i when the upper limit of acidulation of about117% is substantially exceeded unless, of'course, a suflicient amount ofsulfuric acid is employed to convert all of the phosphate values in therock substantially to orthophosphoric acid.

The following examples illustrated this feature of the invention:

Example I 100 grams of particulate phosphate rock ground to a size suchthat about 50% to 60% thereof would pass a 200 mesh screen acidulatedwith about 65 aqueous sulfuric acid in an amount equal to 110% of thatrequisite to convert all of the phosphate values present in the rock tomonocalcium phosphate and to react with the impurities present therein.The acidulated rock so produced was cured for about one-half hour andthen slurried with water employed in an amount equal to 1.1 grams ofWater for each gram of acidnlated rock. The slurry so produced washeated to 60 C. and filtered through a 11 cm. Buckner filter. Thefiltration rate was 13.6 gallons per hour per square foot.

Example 11 The filtration rate was 17.7 gallons per hour per square rfoot.

In the practice of this feature of the invention, at least during thetime the ground phosphate rock and sulfuric acid are admixed, intensiveand thorough agitation of the admixture is necessary. Although it isphysically possible to agitate this freshly prepared admixture for aconsiderable period of time, maximum recovery of phosphorus values intheir water-soluble forms is attainable when the period of agitation iskept as short as possible; provided, however, that the mixing issufficiently long and intense to afford intimate and uniformdistribution of sulfuric acid throughout the phosphate rock mass, andfurther provided the acidified rock is subsequently stored.

In a preferred embodiment of this feature of the invention, the slurry,once having been thoroughly mixed for a short time, generally not morethan a few minutes, is passed onto a continuous and moving belt on whichit is allowed to remain for a period of time, generally about tominutes, sufiicient to permit the soupy material to partially harden orset. The speed of the belt is such as to give a depth of materialsufi'icient to obtain a resultant set of the desired bulk density and toallow the required amount of time for the mix to only partially hardenor set, such that it is not soupy when discharged from the belt. Upondischarge from the belt, the material, having attained its initial set,is transferred to a storage pile. If it is deposited or piled prior tohaving reached its initial set stage, it is often difiicult to removelater from the pile, so much so at times, that explosives may berequired to break it up. When handled as above described, however, it iseasily removed from the pile after storage for from five to fifteen daysby means of mechanical shovels or scoops, or manually. At all timesduring storage, and at the time of removal from storage, the material isporous and friable. The material remains in the storage pile to allowthe reactions to approach equilibrium and to bring the water-solublephosphorus pentoxide in the material up to the maximum within practicallimits. Generally, a two-weak storage time will result in a materialcontaining between about 94% and about 97% water-soluble phosphoruspentoxide, which is desirable at this point in the process in view ofsubsequent process steps. The stored material is then easily pulpcd orslurried with an aqueous medium, since it is not set into a hard mass orlumps which require disintegration by application of explosives or useof hammermills, etc. Sulficient aqueous medium, which may be water orpreviously produced extract or leach solution, is appropriately added sothat the initial resulting slurry contains between about to about ofundissolved solids although more concentrated or more dilute slurriescan be formed. Agitation of the mixture of aqueous medium and acidulatedphosphate rock results in an ultimate slurry containing a liquid phasein the form of a solution containing between about 20% and 33% to 35% byweight of dissolved solids and about 67% to about 80% by weight ofwater. The solid phase of the slurry takes the form of undissolvedsolids which are discarded, which solids contain only about 2.5% of thetotal phosphorus pentoxide values which were originally present in therock and which consist primarily of calcium sulfate. The liquid phase ofthe slurry, after separation from the undissolved solid phase, comprisesessentially an aqueous solution of monocalcium phosphate and a smallamount of phosphoric acid.

Separation of the solids from the extract solution so obtained may becarried out in any convenient and conventional manner such as, forexample, by filtration, countercurrent multistage decantation,preferably at about 60 C., by centrifuging or by use of liquid phasecyclone separators. Increasing the temperature increases the rate ofsettling or separation and, therefore, increases the capacity of thesettling or separation device. However, if material is held above C. forany considerable length of time, some of the water-soluble phosphoruspentoxide precipitates as insoluble dicalcium phosphate.

Aqueous solutions of phosphatic materials so produced are free ofcalcium sulfate and like materials, other than water, and have little orno value as animal feed supple ments or fertilizers.

A threshold problem incident to the production of feed grade dicalciumphosphates from such extracts centers around the reduction of thefluorine content thereof to a degree requisite to the production of feedgrade dicalcium phosphate which has an elemental phosphorus to elementalfluorine Weight ratio of not less than about 100, preferably not lessthan about 200. That aspect of this invention which relates to theproduction from acidulated phosphate rock of a substantially calciumsulfate free extract has particular relevance to feed grade dicalciumphosphate. Eflorts to defluorinate extracts of acidulated phosphate rockin the presence of acid-insoluble materials, including calcium sulfate,are commercially infeasible and result in excessive loss of phosphoruspentoxide values in the calcium sulfate waste product. Accordingly, itis important to remove substantially all of the calcium sulfate from theacidulated rock extract prior to the defluorination thereof.

The prior art has suggested e.g., defluorination of solutions offluorine-containing phosphates, such as acidulated phosphate rockextracts, by addition thereto inter alia, of alkali metal ions,preferably in the form of solutions of alkali metal salts, such assodium and potassium chlorides and the like. Such alkali metaldetluorination processes are ineffective to produce a defluorinated orsolution from which a dicalcium phosphate product of animal feed gradecan be produced. The addition of alkali metal ions to such solutions iseflective only to increase the elemental phosphorus to elementalfluorine weight ratio to a figure substantially below 100, normally lessthan about 50. The addition of excessive amounts of such ions results ina decrease in the phosphorus to fluorine elemental weight ratio in theextract due to the fact that more soluble forms of alkali metalfluoride-containing compounds are produced.

Reference is made to FIGURE 1 which demonstrates that the addition ofsodium hydroxide to wet process 26% orthophosphoric acid obtained by theacidulation of phosphate rock With sulfuric acid in an amount sufficientto convert substantially all of the phosphorus values to orthophosphoricacid, followed by the extraction of the acidulated rock with water andthe removal of calcium sulfate, all in conventional manner, does notachieve substantial defiuorination of such acid. It will be observedthat the sodium ions so introduced are effective to raise the elementalphosphorus to elemental fluorine weight ratio of the acid only to about20, which figure is reached after sodium ions have been added in anamount equal to about 200% of that requisite to form sodiumsilicofluoride with the fluorine present in the acid. It will beobserved that as additional amounts of sodium ions are added, theelemental phosphorus to elemental fluorine weight ratio again decreasesto less than 15. If excessive amounts of sodium ions are added, forexample, in an amount requisite to convert the phosphate present in thesystem from phosphoric acid to monosodium phosphate, the elementalphosphorus to elemental fluorine weight ratio of the resulting systemwill be substantially lower than required for satisfactory calciumdefluorination.

Calcium ion supplying compounds, such as inorganic oxygen-containingbasic calcium compounds, including calcium oxide, calcium hydroxide, andcalcium carbonate, can be employed to effect the requisitedefiuorination of aqueous solutions of fluorine-containing phosphaticmaterials. Such calcium bases react with fluorine and other impuritiespresent, including aluminum and iron, to form fluorine-rich precipitateswhich can be separated as by filtration, decantation, or the like, fromthe defluorinated liquor in which the precipitate is formed.

Effective calcium defluorination of aqueous solutions or"fluorine-containing phosphatic materials through utilization solely ofcalcium bases results in the concurrent precipitation and loss ofexcessive amounts of phosphorus values to a fluorine-rich precipitatewhich is formed. Ac-

cordingly, one feature of this invention is addressed to a process forminimizing the loss of phosphorus pentoxide values to the fluorine-richprecipitate formed when calcium bases or other forms of calcium ions,such as those derived from calcium oxide, calcium hydroxide, and calciumcarbonate and the like, are employed to defluori-nate aqueous solutionsof fluorine-containing phosphatic materials, and particularly extractsor such solutions derived from acidulated phosphate rock.

The quantity of phosphorus pentoxide lost to the fluorineich precipitateformed in the calcium defiuo-rination or" aqueous extracts, such asthose obtained from acidulated phosphate rock, to produce adefluorinated extract having a predetermined minimum fluorine contenthas, to various workers in the art, heretofore been considered to besubstantially independent of the initial fluorine content of the extractprocessed. It has now been discovered that the calcium defluorination ofaqueous solutions of fluorine-containing phosphatic materials isattended by many of the characteristics of systems in which the solidphase or phases are not definite chemical compounds. It can be theorizedthat such calcium defluorination procedures result in the formation offluorine-containing solids crystallized with calcium phosphates in aform analogous to solid solutions of varying composition. The foregoingtheory is offered in the interest of completeness and is not limiting ofor definitive of this aspect of the invention.

In any event, in accordance with this feature of the invention, it hasbeen discovered that the loss of phosphate values to the defluorinationprecipitate in the calcium defluorination of aqueous solutions offluorinecontaining phosphatic materials can be minimized by controllingor adjusting the elemental phosphorus to elemental fluorine weight ratioof the solution to be defluorinated to a value of not less than about15, and preferably to a value within the range of from about 25 to about59, prior to the calcium defluorination step. in general, two basicmethods are available for the adjustment or control of the elementalphosphorus to elemental fluorine weight ratio in the acidulated rockextracts or other aqueous solutions of fluorine-rich-containingmaterials which are to be defluorinated pursuant to the invention.

Pursuant to the first mode of such control, a substantial amount offluorine may be removed from the acidulated rock prior to extractionthereof with an aqueous medium whereby an extract of controlled fluorinecontent is obtained in the first instance. For example, the elementalphosphorus to elemental fluorine Weight ratio of an acidulated phosphaterock extract may be controlled to the desired value of not less thanabout 15 by curing or aging the acidulated rock mix under atmosphericconditions for a period of at least several days, normally at leastabout 5 days, and preferably at least about 14 days or longer, prior toextraction thereof with an aqueous medium. It has been discovered that asufiicient amount of fluorine is eliminated from the acidulated rockduring such aging period to produce an ultimate extract having aphosphorus to fluorine weight ratio of not less than about 15. It willbe appreciated that expedients can be availed of, for example, toshorten the aging time, such expedients taking the form of heating theacidulated rock mixture, or other methods known to the art to expeditechemical reactions.

Alternatively, there may be produced an extract from freshly acidulatedrock, which extract may be thereafter partially defluorinated by meansother than calcium ions prior to the calcium defluorination procedure.For example, alkali metal ions, such as those derived from alkali metalhydroxides, chlorides, sulfates, or the like, including sodium,potassium, and lithium chlorides, bromides, sulfates, hydroxides, andthe like, can be employed to increase the elemental phosphorus toelemental fluorine ratio of such extracts to a figure requisite to thefeasible calcium defiuorination thereof. extract of acidulated phosphaterock can be initially partially defluorinated by the treatment thereofwith the requisite amount of an alkali metal ion, such as an alkali.metal hydroxide or halide, to produce a partially de-- fiuorinatedsolution having an elemental phosphorus to elemental fluorine weightratio of not less than about 15. The defiuorination can thereafter becompleted by treatment of the partially deiiuorinated extract with an appropriate amount of a calcium base material, such as cal-- ciumcarbonate, calcium oxide, or calcium hydroxide, to produce an ultimatedefiuorinated extract in which the elemental phosphorus to elementalfluorine weight ratio is at least about 100.

The particular degree of acidulation of the rock from; which the aqueousextracts of fluorine-containing phosvphatic materials are derived is nota critical feature of this aspect of the invention. Phosphate rock canbe acidulated to any desired degree with any desired mineral acid, suchas sulfuric acid, phosphoric acid, hydrochloric acid, or nitric acid,effective to convert the phosphate values in the rock to water-solubleform. It is contemplated, of course, that the acidulating acid will beemployed in an amount suflicient to convert the predominant amount ofthe phosphate values present in the rock to water-soluble phosphatecompounds, such as monocalcium phosphate or phosphoric acid, or mixturesthereof. The acidulation process described in the preceding portionshereof can appropriately be employed to produce suitable monocalciumphosphate solutions.

The use broadly of calcium ions to defluorinate aqueous solutions offluorine-containing phosphatic materials is known to the art. Such priorart procedures can be employed in the calcium defiuorination step ofthis feature of the invention. In general, such procedures entail theaddition of inorganic calcium bases, such as calcium oxide, calciumhydroxide, or calcium carbonate, preferably in the form of a slurry, tothe solution of phosphatic material to be defluorinated. Reference ismade to Le Baron application Serial No. 424,712, now Patent Serial No.2,889,200, entitled Method of Preparing Defluorinated Material, whichdescribes a preferred method for the calcium defiuorination of aqueousextracts of acidulated phosphate rock. In accordance with the teachingsof that application, such extracts are first adjusted to a phosphoruspentoxide concentration in the range of about 11% to about by weight andthen reacted with calcium carbonate in an amount sufficient to produce apH in the resulting reaction mixture between about 2.3 and 3.0, thereaction time being generall from about to about 120 minutes. Thisinvention, of course, is not restricted to the specific defiuorinationprocedure disclosed in the aforementioned Le Baron application but isembracive of calcium deflourination operations generically of aqueoussolutions of fluorine-containing phosphatic materials in which theelemental phosphorus to elemental fiuorineweight ratio is controlled, ascontemplated by the invention prior to the calcium defluorination step.

Reference is made to FiGU-RE 2 which shows the percentage of phosphatevalues retained in the defiuorinated extract in the calciumdefiuorination of an aqueous extract of acidulatcd rock with bothcalcium oxide and calcium carbonate as a function of the elementalphosphorus to elemental fluorine weight ratios in the treated extracts.The data represented in the figure are those resulting from the calciumdefluorination of the extracts in question to produce in thedefluorinated extracts an elemental phosphorus to elemental'fiuorineweight ratio of 200. The extract solutions employed were wet processphosphoric acid produced by the sulfuric acidulation of phosphate rockfollowed by agitation of the acidulated rock with water and removal ofcalcium sulfate from the slurry so formed, all in conventional mannerweil known to the art, as described, for example, in the book Pi10S-Accordingly, an aqueous.

O a) phoric Acid, etc., by Waggaman, Chapter 12, 2nd edition. Thecalcium oxide and calcium carbonate were added in the form of an aqueousslurry. The extract which was delluorinated was adjusted to a phosphoruspentoxide content of about 18% by weight and the calcium bases wereadded in an amount suflicient to produce a pH in the reaction mixturebetween about 2.3 and 3.0. The contact time between the calcium basesand the extract to be defluorinated was on the order of about 30 to 40minutes. It will be apparent from an examination of FTGURE 2 that thequantity of dicalcium phosphate retained in the defluorinated extractincreases substantially as the ratio of elemental phosphorus toelemental fluorine in the extract to be defiuorinated is increased. Byreference to the curve, it will be observed that the untreated wetprocess phosphoric acid was characterized by an elemental phosphorus toelemental fluorine weight ratio of about 6. Calcium defiuorination ofsuch untreated acid resulted in a loss of more than 55% of thephosphorus pentoxide values to the fluorine-rich precipitate. Adjustmentof the elemental phosphorus to elemental fluorine weight ratio in theextract prior to calcium defiuorination to a value of at least 15 madeit possible to retain in the defiuorinated extract more than of thephosphorus pentoxide values. The control of the elemental phosphorus toelemental fluorine weight ratio in the extracts employed to obtain thedata graphically reflected in FIGURE 2 was effected by the addition ofalkali metal ions to the extract prior to the calcium defiuorination inthe case of the higher elemental phosphorus to elemental fluorine weightratio adjustments, and in some cases, by aging the acidulated rock fromwhich the extract was obtained in the case of the lower of such ratios.

The invention further relates to the production from aqueous extracts ofacidulated phosphate rock of high analysis phosphate fertilizerscontaining available phosphorus pentoxide in both water-soluble andwater-insoluble form. As in the case of feed grade dicalcium phosphate,it is essential in the conversion of such extract solutions to highanalysis phosphate fertilizers, that the Waste product calcium suifatebe effectively and eificiently separated therefrom. It is preferred toproduce such calcium sulfate free extracts by the procedure heretoforedescribed wherein the phosphate values of phosphate rock are extractedprimarily in the form of monocalcium phosphate. This facet of theinvention, however, in at least some of its ramifications, extends tothe production of high analysis fertilizers from alternative types ofaqueous acidulated phosphate rock extracts, as well as from aqueonssolutions or slurries of phosphate materials derived from sources otherthan phosphate rock.

Aqueous extracts of acidulated rock, including such extracts in whichthe phosphate values are present predominantly as nionocalciumphosphate, do not yield a satisfactory commercial fertilizer productmerely by dehydration. Commercially acceptable fertilizer products canbe obtained, however, through addition to such extracts of calciumbases, such as calcium oxide, calcium carbonate, and calcium hydroxide,provided the extracts to which such bases are added are properly andeffectively dehydrated to produce a granular or pellet type fertilizer.

it is generally preferred to employ such calcium bases in an amountrequisite to provide in the resulting mixture :1 calcium oxide tophosphorus pentoxide mole ratio of at least about 0.85, and preferablyat least about 1.0. The upper limit of calcium base addition is afunction of the amount of water-insoluble phosphorus pentoxide desiredin the form of dicalcium phosphate in the final product. An appropriateupper limit for the amount of calcium base material is in an amountrequisite to provide in the resuit ng reaction mixture a calcium oxideto phosphorus mole ratio of not more than about 1.2. A suitable rangefor such calcium base addition is an amount of calcium base requisite toproduce in the extract to which the base is added a calcium oxide tophosphorous pentoxide mole ratio of from about 1.1 to about 1.4.Extracts to which calcium bases have been added in the amount indicated,if properly dried, yield a fertilizer product which is non-hygroscopicand which contains both Water-soluble and water-insoluble forms ofavailable phosphorus pentoxide, containing from about 45% to about 55%by weight of phosphorus pentoxide of which about 20% to about 40% iswater-insoluble.

The proper dehydration of calcium base modified acidulated phosphaterock extracts entails a procedure whereby there is produced from suchcalcium treated extracts a high analysis fertilizer in the form ofgenerally smooth-surfaced, porous pellets comprising a plurality ofgenerally concentric layers of phosphate fertilizer material extendingfrom the center of said pellets outwardly, said layers being ofprogressively increasing diameter. The mode of drying is such that thereis produced in the final pelletized fertilizer product from about toabout by weight of calcium pyrophosphate and from about 2% to about 10%by Weight of calcium metaphosphate.

Reference is made to FIGURE 3 which shows schematically the structure ofthe pelletized fertilizer product of this invention. The schematicdrawing represents a pellet of the fertilizer produce of this invention,one end of which has been removed as by cutting to show a cross sectionthereof. It will be observed that there are a plurality of generallyconcentric layers of fertilizer material extending from the center ofthe pellet outwardly. It may be expected that calcium pyrophosphate andcalcium metaphosphate, which form a part of the fertilizer product ofthe invention, may be positioned or concentrated at the outer surfacesof the various layers, and particularly on the outer surface of thepellet when the pellets are prepared by drying the calcium base treatedextracts in the desired manner, as hereinafter described.

In accordance with this aspect of the invention, there is provided acirculating bed of nuclei about which the pellets of fertilizer productare formed. Such nuclei preferably take the form of solid particlesobtained by drying a calcium base treated aqueous extract of acidulatedrock. Other pellet nuclei, such as particularly phos phate rock, sand,and the like, can be employed, particularly at the initiation of thepellet-forming process. Such circulating bed of pellet nuclei isrepeatedly circulated under a spray of calcium base treated aqueousextract of acidulated rock and dried until there is produced a phosphatefertilizer in the form of a generally smooth-surfaced, porous, pelletcomprising a plurality of generally concentric layers of fertilizermaterial extending from the center of said pellet outwardly and being ofprogressively increasing diameter. The drying of the pellets is carriedout under conditions such that the temperature of the pellets does notat any time exceed a temperature of about 200 C., and the final moisturecontent is not in excess of about 5%, preferably from about 1% to about5%.

In the preferred practice of the invention, the bed of pellet nuclei iscirculated through a direct fired rotary drier, and calcium base treatedacidulated phosphate rock extract is continuously sprayed on therecirculating load within the drier. The drier is maintained at atemperature of about 150 C. to about 250 C., and preferably about 175C., with precautions being taken to insure that the temperature of theproduce does not exceed about 200 C. and to produce a final producthaving a moisture content of not more than about 5%, preferably fromabout 1% to about 5%. It will be appreciated that instead of sprayingthe circulating load of solids with the calcium base treated extractinside the drier, such solids may be sprayed outside the drier andthereafter introduced into the drier as a wet feed.

In the preferred practice of this feature of the invention,approximately one part by weight of the calcium base treated aqueousextract of acidulated rock is added to each 4 to 10 parts by weight ofrecirculating solids,

preferably the addition of calcium base treated extract to the solids isadjusted so that the mixture in the drier does not contain at any timemore than about 15 by weight of moisture. The product discharged fromthe drier is screened for suitable mesh size, and is granular,nondusting, and non-hygroscopic. The form of drier used does notconstitute a critical feature of the invention; driers other than rotarydriers can be employed.

An appropriate calcium base material for use in the production of highanalysis phosphate fertilizers of the invention takes the form of thefluorine-rich precipitate which is obtained in the defiuorinationreaction previously described with reference to the production ofdicalcium phosphate. Such a fluorine-rich precipitate can beappropriately reacted with an extract obtained from acidulated phosphaterock. Such extracts may be obtained from either cured or uncuredacidulated phosphate rock. A particularly suitable extract is oneobtained by acidulating phosphate rock in the manner previouslydescribed with an amount of sulfuric acid equal to from about 112% toabout 117% of that required to convert the phosphate values present inthe rock to monocalcium phosphate, and thereafter without anysubstantial storage time, leaching or otherwise extracting thesolubilized phosphate values from the rock to produce a green or unagedextract. Normally such an extract can be prepared by processing theacidulated phosphate rock within a few minutes, normally within about 15to about 90 minutes, after the acid and rock have been combined. Theresulting extract is rich in phosphoric acid and may be expected to becharacterized by a calcium oxide to phosphorus pentoxide mole ratio offrom about 0.18 to about 0.30. Other types of aqueous extracts ofacidulated phosphate rock can, of course, be employed.

The fluorine-rich precipitate produced incident to the production ofdicalcium phosphate, as above described, can be reacted with such agreen extract in an amount requisite to provide the desired calciumoxide to phosphorus pentoxide mole ratio in the ultimate fertilizerproduct. The reaction mixture or slurry so produced is formed into apelletized fertilizer product in the same manner as previously describedto produce a multi-layered, pelletized, high analysis phosphatefertilizer containing from about 45% to about by weight of availablephosphorus pentoxide. Such a product contains from about 30% to about50% by weight of monocalciurn phosphate, from about 5% to about 15% byweight of dicalcium phosphate, from about 0.5% to about 2% by weight oftricalcium phosphate, from about 2% to about 10% by weight of calciummetaphosphate, from about 5% to about 10% of calcium pyrophosphate, fromabout 3% to about 8% by weight of free phosphoric acid, and minorpercentages of fluorine, aluminum, and iron containing materialsaggregating not more than about 10% by weight of the fertilizer product.

The use of the fluorine-rich preci itate formed in the manufacture ofdicalcium phosphate in the production of a high analysis phosphatefertilizer product is of particular significance in that embodiment ofthe invention which contemplates an integrated process for thesimultaneous or concurrent production of high analysis fertilizers andfeed grade dicalcium phosphate. Such a process is described in greaterdetail in the aforementioned copending Le Baron application Serial No.312,519.

A specific embodiment of the invention, as addressed to the productionfrom phosphate rock of a granular, nonhygroscopic, free-flowingfertilizer containing a high percentage of phosphorus pentoxide which issubstantially completely available and which is present in bothwatersoluble and water-insoluble forms, entails (1) continuously mixingand agitating phosphate rock ground to a particle size such that about50% to about 85% thereof will pass through a 200 mesh screen with toaqueous sulfuric acid, said sulfuric acid being employed in an amountequal to about 112% to about 117%, preferably about 115%, of thatrequired to form monocalcium phosphate from the phosphatic materialscontained in said rock and to react with the impurities contained insaid rock, said mixture being agitated for not more than about 4minutes, preferably not more than about 1 minute; (2) discharging theagitated mixture onto a continuously moving conveys-rand maintainingsaid mixture on said conveyor for about to minutes to permit saidmixture to initially set; thereafter discharging said mixture ininitially set condition onto a curing pile and storing said mixture insaid curing pile for several days, preferably about 5 to 15 days, toproduce a cured phosphate material which is friable and porous andwhich, without prior mechanical disintegration, is easily slurried withan aqueous medium; (3) forming an initial aqueous slurry of said curedphosphate material containing about to about solids; (4) agitating saidslurry for a short period of time to form a liquid phase conta ningabout 20% to about 33% dissolved solids and about 67% to about 80%water; (5 heating said slurry to an elevated temperature not in excessof 60 C. and separating said liquid phase from the non-dissolved solidsmaterials contained in said slurry and adding to said separated liquidphase a small amount of an inorganic calcium base, such as limestone;(6) thereafter introducing said liquid phase into a load, recirculatingthrough a drier, of a particulate solid phosphate material resultingfrom the prior dehydration of similarly obtained liquid phasecompositions, said liquid phase being added to said recirculating loadin an amount such that the resulting combination does not contain morethan about 15% of moisture, there being employed one part of said liquidphase for each 4 to 10 parts of solids in said load; ('7) introducingsaid combination into said drier while said drier is operated at atemperature of about 150 C. to about 230 C. and maintaining saidcombination in said drier for a time period sufiicient to effectreduction of the moisture content thereof to between about 1% and about5% by weight to produce at a temperature not in excess of 200 C. agranular, nonhygroscopic, free-flowing product containing about to about58% phosphorus pentoxide, of which about 29% to about 40%, preferablyabout 35%, by weight is waterinsoluble, said available phosphoruspentoxide constituting substantially the entire amount of phosphoruspentoxide present in said product.

It will be appreciated that the acidulated phosphate rock may be simplydischarged from the acidulating vessel into a cured or storage pilewithout necessarily being transported to such pile by means of a movingconveyor, although it is desirable to the end that a fria le, curedproduct may be obtained, to permit the mixture to initially set and thendisturb the initially set condition. Further, the specific embodiment ofthe invention, as described in detail above, may be practiced with anextract of freshly acidulatcd rock rather than an extract of curedrocic.

In the practice of this invention for the production of a most desirableform of acid-elated rock mixes, the rock is ground to a particle sizesuch that substantially the entire amount of the rock will pass a 14mesh screen with about 50% to about 80% by weight being of a particlesize requisite to pass a 200 mesh screen.

The following examples are illustrative of the best modes presentlyknown to the applicants for practicing the various features of theinvention.

Example III 12 the point of discharge of slurry-onto the belt. The beltlength and its speed were such that the mixture remained on the beltabout 20minutes. The discharge from the belt was stored in a pile forabout 14 days.

The stored material was then removed from storage, broken up andsutficient water added to give a slurry of about 35% undissolved solids.The slurry was subjected to four steps of continuous counter-currentdecantation followed by a single filtration to produce a leach solutioncontaining about 30% dissolved solids being of approximately 32 as.gravity. The discarded tailings contained about 2.5% ot the 30% totalphosphorus pentoxide, only about one-half of which is available. Thisextract was delivered to the storage tank 37, and had an elementalweight P/F of about 20.

A second acid mix was prepared utilizing the same proportions, but wasnot sent to storage. The green or unagcd mix was delivered directly tothe countercurrent decantation operation to produce a'leach solutioncontaining approximately the same percentage of dissolved solids as theextract from the aged superphosphate. This solution was delivereddirectly to storage tank 33. The extract from aged supcrphosphate wasfurther processed by adding thereto approximately 6 parts by weight oflimestone per parts by weight of extract solution. It will be recognizedthat other materials capable of reacting in the same molecularproportions may be substituted for limestone, such as calcium oxide. Themixing of these proportions of ingredients results in the precipitationof the major portion of the fluoride present in the extract to produce afiltrate having approximately 15% phosphorus pentoxide, 0.04% fluorine,and a calcium oxide to phosphorus pentoxide ratio of approximately 0.9.To this liltrate is added approximately 12 parts by weight of comminutedlimestone per 100 parts of defiuorinated extract. The result of thereaction of these ingredients is the precipitation of a material whichis predominantly dicalcium phosphate. The dicalcium phosphate solids arefiltered off and show a recovery of approximately 99% of the phosphatespresent in the extract solution. The dicalcium phosphate is dried andthe product contains approximately 20% phosphorus, and an elementalweight P/F in excess of 100.

The filter cake containing the precipitated fluorides was mixed withextract from green superphosphate from storage tank 38 in theproportions of approximately 11 pounds of cake per 100 pounds by weightof green extract solution. The resultant slurry is dried in a rotarykiln and screened to produce a -3+12 mesh product containingapproximately 56% available phosphorus pentoxide, the drying beingeffected as in Example II.

Example 1V About 10 tons per hour of phosphate rock, ground to 50%passing through a 200 mesh screen and of about 67% bone phosphate oflime analysis, was mixed with about 6 tons per hour of about 96%sulfuric acid added as 50-60 Be. aqueous solution. The mixture wasthoroughly agitated for about one minute, after which it was dischargedonto a continuous belt. The belt length and its speed were such that themixture remained on the belt about twenty minutes. The discharge fromthe belt was stored in a pile for about fourteen days. The storedmaterial was then removed from storage, broken up, and sufficientsolution from the process (previously prepared leach solution plus addedwater) added to give a slurry of about 35% non-dissolved solids.Continuous multistage countercurrent decantation (for example, using atray washer), produces a leach solution containing about 30% dissolvedsolids. This is the primary leach or phosphate solution. This solutionis freed of non-dissolved solids by centrifuging, filtering, or by theuse of cyclone separators, and has a P/E weight ratio of about 20.

It is then further processed by adding about 0.45 ton per hour of marbleflour or other suitable limestone to the leach solution. The solution iscontinuously dried and granulated in a direct fired rotary kiln at atemperature not exceeding about 200 C. In the drying of the productthere was provided within the rotary kiln a recirculating bed ofpreviously dried material such that there was built up a pelletizedproduct in the form of a series of layers of high analysis phosphatefertilizer extending from the center of said pellets outwardly. The bedof material which was recirculated through the drier was continuouslyscreened and a desired portion of properly sized pellets separatedtherefrom. Undersized material was recirculated and oversized materialwas ground and recirculated as a part of such bed. The final pelletizedproduct contained from about 30% to about 50% by weight of monocalciumphosphate, from about to about by weight of dicalcium phosphate, fromabout 0.5% to about 2% by weight of tricalcium phosphate, from about 2%to about 10% by weight of calcium metaphosphate, from about 5% to about10% by weight of calcium pyrophosphate, and from about 3% to about 8% byweight of free phosphoric acid. The product was non-hygroscopic andcontained from about 55% to about 56% by weight of available phosphoruspentoxide of which about 30% to about 35% by weight was water soluble.

In place of using about 0.45 ton of marble flour, about 0.25 ton ofcalcium oxide, or its equivalent in calcium hydroxide alternatively, maybe used. This product also contains about 55-56% phosphorus pentoxide inavailable form. Also, in producing this product (i.e., the 5556%available phosphorus pentoxide) substantially equivalent molarquantities of calcium oxide or calcium hydroxide, as compared with themolar quantities of marble flour, can be employed. Dolomitic limestoneor other similar limestone may be employed in place of marble flour. Inthis instance, the final product will be found to contain a lowerfluorine content than when ordinary limestone is used. Mixtures ofquicklime or hydrated lime with limestone are also suitable for use.

Example V An extract of green or unaged acidulated phosphate rock wasprepared in the same manner as described in Example III, particularlywith reference to the treatment of the second acid mix there referredto. The extract so obtained was characterized by an elemental phosphorusto elemental fluorine weight ratio of about 6. The extract so producedwas treated by contact thereof with superheated steam in a mannergenerally similar to that described in Patent No. 2,165,000 to increasethe elemental phosphorus to elemental fluorine mole ratio to a value ofabout 25. The extract was then reacted with calcium carbonate in anamount requisite only to substantially completely precipitate thefluorine-containing materials present therein. The fluorineprecipitation reaction was carried out at a pH of about 2.5 for a timeperiod of about 45 minutes and the precipitate so formed separated fromthe mother liquor. About 85% of the phosphate values originally presentin the extract were retained in the mother liquor which was thereafterreacted in conventional manner with additional calcium carbonate toproduce a feed grade dicalcium phosphate precipitate which was recoveredand dried.

In lieu of steam, alkali metal ions, for example in the form of aqueoussolutions of alkali metal salts, such as sodium and potassium chloride,sulfate, and the like, can be employed to raise the elemental phosphorusto elemental fluorine weight ratio of such extracts to a value of notless than 15.

The term feed grade dicalcium phosphate is employed herein as embraciveof dicalcium phosphates having a fluorine content sufliciently low to beacceptable for use as animal feed supplements.

We claim:

1. A non-dusting, non-hygroscopic, high analysis phosphate fertilizer inthe form of porous pellets comprising a plurality of generallyconcentric layers of phosphate fertilizer material extending from thecenter of said pellets outwardly, said layers being of progressivelyincreasing diameter, each of said layers being dried on its surface soas to exhibit a non-hygroscopic behavior, there being present in saidfertilizer product from about 5% to about 15 by weight of calciumpyrophosphate and from about 2% to about 10% by weight of calciummetaphosphate, said fertilizer product containing from about 45% toabout 60% by weight of phosphorus pentoxide of which about 20% to about40% by Weight is water-soluble, the calcium pyrophosphate and calciummetaphosphate content of the said fertilizer product being concentratedat the hard, dried, non-hygroscopic surface of each of said layers.

2. A process for producing a multilayer high analysis, granular,non-hygroscopic phosphate fertilizer which comprises acidulatingparticulate phosphate rock with an amount of sulfuric acid requisite toconvert the phosphate values in said rock substantially entirely towatersoluble form, slurrying the acidulated rock with water to provide aliquid phase in the form of an aqueous solution of said water-solublephosphate values and a solid phase of predominantly calcium sulfate,separating said liquid phase solution from said solid phase calciumsulfate, adjusting the calcium oxide to phosphorus pentoxide mole ratioin said liquid phase solution to a value of not less than about 0.85with a calcium base, thereafter spraying said liquid phase solution ontoa load of particulate pellet nuclei and drying the coated nuclei soformed in a drier, thus producing dried coated nuclei having hardnon-hygroscopic surfaces, recirculating these dried coated nuclei to thespraying zone and forming a second sprayed coating thereon, drying thethus coated nuclei, and continuing said recirculating and drying for atime period requisite to produce a pellet of the desired size in theform of a plurality of concentric layers of phosphatic fertilizermaterial extending from the center of said pellets outwardly, each ofsaid concentric layers being characterized by a hard, non-dusting,non-hygroscopic surface.

3. The process of claim 2 wherein said bed of pellet nuclei takes theform of solid material obtained by the drying of said liquid phasesolution, the calcium oxide to phosphorus pentoxide mole ratio of whichhas been adjusted to a value of not less than about 0.85.

4. The process of claim 3 wherein said liquid phase phosphate solutionafter adjustment of the calcium oxide to phosphorus pentoxide mole ratiothereof to a value of not less than about 0.85 is sprayed on saidrecirculating load in an amount such that the moisture content of theload subsequent to said spraying does not exceed about 15 by weight.

5. The process of claim 3 wherein said drier is operated at atemperature of from about C. to about 230 C.

References Cited by the Examiner UNITED STATES PATENTS Re. 19,750 11/35Billings et al. 23-313 1,239,221 9/17 Rodman 23-313 1,761,400 6/30Liljenroth 71-40 2,041,088 5/ 36 Pfirrmann 71-64 2,142,944 1/ 39Kerschbaurn 23-106 2,174,614 10/39 Bornemann et a1 23-106 2,244,158 6/41 Hubbard et al 23-313 2,709,649 5/ 55 Le Baron 71-40 2,739,886 3/56Pacer 71-40 2,759,795 8/56 Archer 71-40 DONALL H. SYLVESTER, PrimaryExaminer.

MAURICE A. BRINDISI, GEORGE D. MITCHELL,

A. LOUIS MONACELL, Examiners.

1. A NON-DUSTING, NON-HYGROSCOPIC, HIGH ANALYSIS PHOSPHATE FERTILIZER INTHE FORM OF POROUS PELLETS COMPRISING A PLURALITY OF GENERALLYCONCENTRIC LAYERS OF PHOSPHATE FERTILIZER MATERIAL EXTENDING FROM THECENTER OF SAID PELLETS OUTWARDLY, SAID LAYERS BEING OF PROGRESSIVELYINCREASING DIAMETER, EACH OF SAID LAYERS BEING DRIED ON ITS SURFACE SOAS TO EXHIBIT A NON-HYGROSCOPIC BEHAVIOR, THERE BEING PRESENT IN SAIDFERFILIZER PRODUCT FROM ABOUT 5% TO ABOUT 15% BY WEIGHT OF CALCIUMPYROPHOSPHATE AND FROM ABOUT 2% TO ABOUT 10% BY WEIGHT OF CALCIUMMETAPHOSPHATE, SAID FERTILIZER PRODUCT CONTAINING FROM ABOUT 45% TOABOUT 60% BY WEIGHT OF PHOSPHORUS PENTOXIDE OF WHICH ABOUT 20% TO ABOUT40% BY WEIGHT IS WATER-SOLUBLE, THE CALCIUM PYROPHOSPHATE AND CALCIUMMETAPHOSPHATE CONTENT OF THE SAID FERTILIZER PRODUCT BEING CONCENTRATEDAT THE HARD, DRIED, NON-HYGROSCOPIC SURFACE OF EACH OF SAID LAYERS.
 2. APROCESS FOR PRODUCING A MULTILAYER HIGH ANALYSIS, GRANULAR,NON-HYGROSCOPIC PHOSPHATE FERTILIZER WHICH COMPRISES ADIDULATINGPARTICULATE PHOSPHATE ROCK WITH AN AMOUNT OF SULFURIC ACID REQUISITE TOCONVERT THE PHOSPHATE VALUES IN SAID ROCK SUBSTANTIALLY ENTIRELY TOWATERSOLUBLE FORM, SLURRYING THE ACIDULATED ROCK WITH WATER TO PROVIDE ALIQUID PHASE IN THE FORM OF AN AQUEOUS SOLUTION OF SAID WATER-SOLUBLEPHOSPHATE VALUES AND A SOLID PHASE OF PREDOMINANTLY CALCIUM SULFATE,SEPARATING SAID LIQUID PHASE SOLUTION FROM SAID SOLID PHASE CALCIUMSULFATE, ADJUSTING THE CALCIUM OXIDE TO PHOSPHORUS PENTOXIDE MOLE RATIOIN SAID LIQUID PHASE SOLUTION TO A VALUE OF NOT LESS THAN ABOUT 0.85WITH A CALCIUM BASE, THEREAFTER SPRAYING SAID LIQUID PHASE SOLUTION ONTOA LOAD OF PARTICULATE PELLET NUCLEI AND DRYING THE COATED NUCLEI SOFORMED IN A DRIER, THUS PRODUCING DRIED COATED NUCLEI HAVING HARDNON-HYDROSCOPIC SURFACES, RECIRCULATING THESE DRIED COATED NUCLEI TO THESPRAYING ZONE AND FORMING A SECOND SPRAYED COATING THEREON, DRYING THETHUS COATED NUCLEI, AND CONTINUING SAID RECIRCULATING AND DRYING FOR ATIME PERIOD REQUISITE TO PRODUCE A PELLET OF THE DESIRED SIZE IN THEFORM OF A PLURALITY OF CONCENTRIC LAYERS OF PHOSPHATIC FERTILIZERMATERIAL EXTENDING FROM THE CENTER OF SAID PELLETS OUTWARDLY, EACH OFSAID CONCENTRIC LAYERS BEING CHARACTERIZED BY A HARD, NON-DUSTING,NON-HYGROSCOPIC SURFACE.